The invention relates to a transmission, in particular for motor vehicles, as generically defined by the preamble to claim 1. One such transmission is described in the article entitled xe2x80x9cA development of Toyota Hybrid Systemxe2x80x9d (Technical Review, Vol. 47, No. 2, Apr. 2, 1998). In the known transmission for a hybrid vehicle, a single planetary gear is provided. Its ring gear connected to the output shaft is coupled to a first electric machine, while the sun wheel is connected to a second electric machine. Via the planet carrier connected to the drive shaft or crankshaft of the internal combustion engine, a driving moment is introduced into the planetary gear. By varying the rotary speed of the sun wheel coupled to the second electric machine, an arbitrary rotary speed can be established for a given planet wheel rotation, generated by the engine, at the ring gear, and as a result arbitrary gear ratios between the ring gear and thus the output shaft and the engine can be attained. The torques are always divided by a fixed ratio, by means of the planetary gear, among the engine, the output shaft, and the second electric machine. For a given torque of the engine and a certain gear ratio, the rotary speed and the requisite torque at the second electric machine are thus fixed. If the requisite rotary speed at the second electric machine changes, then its electrical power varies as well. In the generator mode of the second electric machine, the power thus generated can be output to the first electric machine again, while in the motor mode of the second electric machine the process is correspondingly reversed. This process is known as electrical power branching. A disadvantage of such power branching, however, is the incident losses.
For this reason, the so-called SEL transmission has been proposed (P. Tenberge, W. Hofmann: Mechanisch-elektrische Fahrzeuggetriebe im Vergleich [A Comparison of Mechanical-Electrical Vehicle Transmissions], VDI-Bericht Nr. 1393, VDI-Verlag Dxc3xcsseldorf 1998, pages 551 ff.). Here, downstream of the planetary set of a planetary gear there is an additional three-stage gear that reduces the spreading of the rotary speed of the electric machine. A disadvantage is the relatively complicated gear mechanics and the requisite hydraulics for shifting the lamination clutches and brakes.
The transmission of the invention, especially for motor vehicles, having the characteristics of the body of claim 1 has the advantage over the prior art that it is relatively simply constructed in mechanical terms and has good efficiency. This is attained according to the characteristics of the body of claim 1 in that because of the two planetary trains each coupled to one electric machine, it is possible to reduce the electrical power flow of the two electric machines such that in the generator mode, for instance, only the power required by the on-board electrical system is generated by the two electric machines.
Further advantages and advantageous refinements of the transmission according to the invention, in particular for motor vehicles, will become apparent from the dependent claims and the description.
By coupling the two electric machines each to one gear shaft and by using planetary trains, conventional clutches can be dispensed with. The two electric machines furthermore replace a starter otherwise needed for the starting the engine and a generator and synchronizing devices for shifting individual gears. Moreover, by the use of two gear shafts, shifting the gears can be attained without interrupting the tractive force. By means of the two electric machines, it is furthermore possible to achieve an infinitely graduated step-up in the range between two gear stages.